The engine in an automotive vehicle is typically cooled by liquid coolant which is pumped through a liquid-to-air heat exchanger, or radiator. Due to the difference in density between coolant and air, the radiator is typically relatively narrow in width, but has a large face area through which the cooling air passes. Other vehicular heat exchangers, such as a condenser for the air-conditioning system, have similar configuration and are often cooled in series with the radiator.
The location of these heat exchangers is typically the front of the vehicle, behind openings in the vehicle body, so high pressure due to forward motion of the vehicle can cause air to move through them. However, in order to assure that sufficient air moves through the heat exchangers when the cooling requirements are severe, or when the vehicle is not moving, a fan assembly is fitted either upstream or downstream of the heat exchangers.
The fan assembly typically includes a fan and a shroud which surrounds the fan and guides air between the heat exchanger and the fan. The fan is typically driven by an electric motor supported by a bracket which is attached to, or integral with, the shroud. Due to under-hood space constraints, the shroud must in general be of minimum depth while at the same time covering a large area of heat exchanger surface. Because of this, much of the cooling air approaches the fan from essentially the (negative) radial direction, and must turn almost 90 degrees if it is to flow through the tip region of the fan.
If it fails to turn sufficiently, it will separate from the shroud surface, and compromise the efficiency and acoustic performance of the fan.
Another constraint on the fan design is that its noise be acceptable to the customer. Fan noise includes both broadband noise and tones, the latter being generated by the fan""s interacting with a non-axisymmetric inflow. One way of minimizing these tones is to incorporate skew in the blade design. Skewed blades can, however, have structural problems which radial blades do not encounter.
There are many other constraints on the design of the fan assembly. One requirement is that the fan and shroud be inexpensive to manufacture. For this reason it is typically a plastic injection-molded part. Clearances between the fan and shroud must accommodate manufacturing tolerances as well as deflections of the parts in service. These deflections include long-term creep, and depend on time, temperature, and humidity. Fan deflections arise from centrifugal and aerodynamic forces and include components in both the radial and the axial directions. The fan assembly must be designed in such a way that the fan does not contact the shroud at any time, and yet have a sufficiently small clearance gap that leakage between the fan and the shroud does not overly compromise efficiency or noise. Two types of fans have been used for this application, differing in the nature of the clearance gap through which leakage occurs.
One type of fan is a free-tipped fan, where the clearance gap is between the shroud and the ends of the rotating blades. This type of fan typically has blades which are almost radial in configuration, with only a small amount of skew. Typically the blades have a constant-radius tip shape, so that only a radial deflection, which is minimized by their almost-radial configuration, can cause contact with the shroud. FIG. 1a shows a typical free-tipped engine-cooling fan.
The second type of fan is a banded fan, the blade tips of which are attached to a rotating band. The clearance gap through which recirculation takes place is between the rotating band and the shroud. One advantage of this configuration is that the leakage flow can be minimized by use of various leakage control devices (U.S. Pat. No. 5,489,186). Another advantage is that the band can provide structural support for skewed blades (U.S. Pat. Nos. 4,569,631, 4,569,632), minimizing their deflection.
Both of these fan types have disadvantages.
The efficiency of free-tipped fans depends strongly on the tip gap. Air moves around the blade tip from the pressure side to the suction side, thereby reducing the pressure difference across the blade in the tip region and generating a concentrated tip vortex. This vortex is a loss mechanism, and can be a source of noise. A configuration such as that shown in FIG. 1b minimizes the tip gap, but at the expense of flow separation due to the small inlet radius on the shroud barrel. FIG. 1c shows a more typical free-tip fan assembly, where separation is minimized by allowing the forward portion of the blade tip to extend into the fan plenum and employing a more generous inlet radius. This configuration, however, has higher tip leakage losses, since small tip gaps are maintained only at the rearward portion of the blade tips. Free-tipped fans tend to be noisier than banded fans, particularly at more resistive operating points. Stall tends to be more extreme and more sudden for these fans.
Although banded fans have reduced tip clearance losses relative to free-tipped fans, they have the additional viscous losses of the rotating band. These losses are particularly severe at lightly-loaded operating points, where the fan speed is relatively high for the pressure and flow developed. Such operating points are common in automotive applications, since they allow the use of inexpensive low-torque motors. Another source of parasitic loss for a banded fan is flow separation at the band. Due to molding requirements, the inner surface of the band must be essentially cylindrical over the axial extent of the blades, as shown in FIG. 1d. A lip is usually added to the front of the band, but it is of necessity of limited extent, due to the tight space requirements. Flow separation is often the result. The rotating band also leads to some noise and vibration problems. Any axial run-out of the band causes a large couple imbalance which can lead to vibration problems in the vehicle. Also, the large moment of inertia of a banded fan prolongs the time during which the fan coasts down when the fan is de-powered. The coast-down process can lead to objectionable noise in the vehicle. In addition to these performance issues, banded fans can be more expensive to manufacture than free-tipped fans. The mass of the band at a large radius makes a banded fan more likely to require a separate balancing operation than a free-tipped fan. A banded fan requires the use of more material than would be required for a free-tipped fan, and the presence of knit lines in the band requires the use of more expensive material than might otherwise be used.
One object of the invention is to maximize the efficiency of an automotive engine-cooling fan assembly by minimizing leakage between the fan and the shroud.
Another object is to maximize the efficiency of the fan assembly by minimizing flow separation.
Another object is to minimize the noise generated by the fan.
Another object of the invention is to provide a low-cost assembly by minimizing the amount and cost of the plastic material used in its manufacture.
Another object is to minimize the static and couple imbalance of the fan, and thereby reduce the cost of balancing the fan and the amount of vibration in the vehicle.
Another object is to minimize the moment of inertial of the fan in order to shorten the coast-down process when the fan is de-powered.
The present invention is an un-banded automotive engine-cooling fan and shroud assembly. The shroud has a barrel with a flared inlet and at least a portion of each blade tip conforms to the shape of this inlet. The radius of the blade tip is larger at the upstream end of the conforming portion than at the downstream end of this portion.
In a preferred embodiment, the entire blade tip conforms to the shape of the shroud inlet. Also in a preferred embodiment, the clearance gap between the blade tip and the shroud is approximately constant. Because the tip gap is maintained at its minimum value over substantially the entire blade tip, tip clearance losses and fan noise are minimized. In addition, the large inlet flare allowed by this design minimizes flow separation. This also maximizes fan efficiency and minimizes noise.
In one particular embodiment, the blade tip extends upstream of the portion of the blade tip which conforms to the shroud flare. In this embodiment, the axial extent of this upstream portion is less than approximately 0.3 times the axial extent of the blade tip.
The shroud barrel downstream of the flared inlet may be approximately cylindrical. In one embodiment the blade tip extends downstream of the downstream end of the shroud flare. In this embodiment, the axial extent of this downstream portion is less than approximately 0.5 times the axial extent of the blade tip.
In the preferred embodiment, the radius of the shroud barrel at the axial position of the blade trailing edge does not exceed the minimum radius of the shroud barrel by more than 0.02 times the fan diameter. References to shroud radii refer to the radius of the air passage inside the shroud.
In one embodiment, the shroud barrel may step inward downstream of the trailing edge of the blade tip.
In yet another embodiment, the shroud barrel is relatively short, in that the distance between the termination of the shroud barrel and the trailing edge of the blade tip is less than approximately 0.5 times the axial extent of the blade tip. In a preferred embodiment, this distance is less than approximately 0.3 times the axial extent of the blade tip.
The invention also features blade geometry that minimizes deflection at the blade tip. In one embodiment the fan is radial-bladed, and the tips are raked forward by less than 3 percent of the fan diameter. In a preferred embodiment, the fan is skewed. Preferably, the fan has a forward rake angle in regions where it is either forward-swept or it is back-swept less than approximately 5 degrees, and it has a rearward rake angle where it is back-swept by more than approximately 15 degrees.
In a preferred embodiment, the fan is swept forward near the hub and backward near the blade tips, and has forward rake angle near the hub and rearward rake angle near the tips
In another embodiment, the fan is swept backward near the hub and forward near the blade tips, and has rearward rake angle near the hub and forward rake angle near the blade tips.
In a preferred embodiment, the flare shape is approximately elliptical, the distance between every point on the surface of the flared inlet and a corresponding point on an approximating ellipse being less than 0.5 percent of the fan diameter. In a preferred embodiment the approximating ellipse is oriented so as to have axial and radial semi-axes, and has an axial semi-axis approximately 0.5 to 2.0 times the axial extent of the blade tip, and a radial semi-axis approximately 0.4 to 1.0 times the axial semi-axis. In the preferred embodiment the axial semi-axis is between 0.04 and 0.14 times the fan diameter, and the radial semi-axis is between 0.02 and 0.11 times the fan diameter.
In a preferred embodiment, the radius of the upstream end of the conforming portion of the blade tip is between approximately 2% and 15% greater than the radius of the downstream end of the conforming portion of the blade tip.
In a preferred embodiment, the minimum clearance between the blade tip and the shroud is between 0.007 and 0.02 times the fan diameter. The axial distance measured at a constant radius between the blade leading edge and the shroud is between approximately 0.011 and 0.034 times the fan diameter.
In the preferred embodiment, the distance between each point on a curve in the meridional plane swept by the conforming portion of the blade tip and a corresponding point on an approximating ellipse is less than 0.5 percent of the fan diameter. In the most preferred embodiment the ellipses approximating the shapes of the flared inlet and the blade tip are oriented so as to have axial and radial semi-axes, and the difference between the axial semi-axes of the two ellipses is equal to or greater than the difference between the radial semi-axes.
In the preferred embodiment, the leading edge of the fan tip is no more than 0.04 fan diameters downstream of the upstream edge of the shroud flare.
In the preferred embodiment the blade chord at the tip is approximately 0.2 to 0.4 times the fan diameter.
In one embodiment the fan assembly is mounted downstream of a heat exchanger. In the preferred embodiment, the shroud incorporates a plenum, which covers an area of the heat exchanger face, which is at least 1.5 times the disk area of the fan. This embodiment benefits particularly from the large inlet flare, which is a feature of this invention. The flow from the plenum region has a large radial component as it approaches the fan barrel, and separation is likely in the absence of such a flare.
In another embodiment, the fan assembly is mounted upstream of a heat exchanger.
In the preferred embodiment, the fan and the shroud are made of injection-molded plastic. In the most preferred embodiment the shroud is molded as a single part.